The present invention relates to a semiconductor memory device, and more particularly to a semiconductor memory device having a one-transistor type memory cell in which one transistor and one capacitor are connected in series.
A dynamic type RAM (Random Access Memory) has recently been put into extensive practical use which employs a one-transistor type memory cell in which one insulated gate type field effect transistor (hereinbelow, simply termed "MISFET") and one capacitor are connected in series. A theme of the manufacturing technology of this dynamic RAM is to reduce to the utmost the area occupied by the capacitors of a large number of memory cells which are formed on a semiconductor substrate. As a solution for reducing the area occupied by the capacitors, higher permittivity material has been tried for a dielectric material which constitutes the capacitors. Heretofore, a silicon oxide film has been common as the dielectric material put into pratical use. It has been considered to replace this material with a silicon nitride film of high permittivity. Silicon nitride film has a permittivity which is about double that of silicon oxide film, and, because of this, a remarkable decrease in the occupied area relative to that of the conventional capacitor employing silicon oxide film can be expected.
As the result of experimental research, however, the inventor has discovered the problem that, when a capacitor is formed on the surface of a semiconductor substrate by employing a silicon nitride film as the dieletric material between the two electrodes of the capacitor, the surface state of the semiconductor substrate surface changes to vary the capacitance value of the capacitor, in accordance with the magnitude and polarity of a voltage applied across the electrodes. More specifically, the inventor's experimental research has revealed that, in a case where a silicon oxide film is formed on the surface of a semiconductor substrate and is further overlaid with a silicon nitride film so as to use both films as the dielectric film of a capacitor, unfavorable carriers are accumulated in a trap region formed in the dielectric film. The charges accumulate in accordance with the magnitude and polarity of an applied voltage, and they change the state of the semiconductor substrate surface. This causes the capacitance value of the capacitor to undesirably vary with time. The variation in the capacitance value of the capacitor forms the cause of affording a soft error or malfunction to a memory device, and results in limiting the lifetime during which the memory device performs normal operations.